This invention relates generally to the field of metal alloys useful for metal plating. In particular, the present invention relates to electrolyte compositions for depositing a tin alloy on a substrate and to methods of depositing a tin alloy on a substrate. The invention further relates to methods of forming interconnect bumps on a semiconductor device. Particular applicability can be found in semiconductor device packaging in the formation of interconnect bumps on semiconductor devices.
Tin and tin-lead alloy deposits are useful for the electronics industry, particularly in the manufacture of printed wiring boards, electrical contacts and connectors, semi-conductors, electrical conduit and other related parts where the inherent properties of these deposits are necessary. Of the various electronic applications, there is a current focus in the semiconductor manufacturing industry on wafer-level-packaging (WLP). With wafer-level-packaging, IC interconnects are fabricated en masse on the wafer, and complete IC modules can be built on the wafer before it is diced. Benefits gained using WLP include, for example, increased I/O density, improved operating speeds, enhanced power density and thermal management, and decreased package size.
One of the keys to WLP is the build up of flip-chip conductive interconnect bumps on the wafer. These interconnect bumps serve as electrical and physical connections of the semiconductor components to a printed wiring board. Several methods of forming interconnect bumps on semiconductor devices have been proposed, for example, solder plate bumping, evaporation bumping, conductive adhesive bonding, stencil printing solder bumping, stud bumping, and ball placement bumping. Of these techniques, it is believed that the most cost effective technique for forming fine pitch arrays is solder plate bumping, which involves a combination of a temporary photoresist plating mask and electroplating. This technique is being rapidly adopted as full-area interconnect bump technology for high value-added assemblies such as microprocessors, digital signal processors, and application specific integrated circuits.
Electroplating methods for depositing tin, tin-lead and other tin-containing alloys are well known and many electrolytes have been proposed for electroplating such metals and/or alloys. For example, U.S. Pat. No. 4,880,507 to Toben et al. discloses electrolytes, systems and processes for depositing tin, lead or a tin-lead alloy. The electronics industry has recently been in search of alternatives to tin-lead in light of the toxic properties of lead and the resulting current worldwide activities to ban its use. Suitable replacements for tin-lead alloys should possess the same or sufficiently similar properties to the tin-lead for a given application. Once a suitable replacement material has been found, development of an electroplating process capable of depositing such material to impart the desired properties can be a challenge.
It is desired that the composition of the deposits be effectively controlled to prevent melting of the material at too high or too low a temperature for a given application. Poor compositional control can result in either a temperature too high for the components being treated to withstand or, on the other extreme, incomplete formation of the solder joint.
Difficulties associated with co-deposition of lead-free tin alloys by electroplating arise when the materials being deposited have significantly different deposition potentials. Complications can arise, for example, when attempting to deposit alloys of tin (−0.137 V) with copper (0.34 V) or silver (0.799 V). To allow for co-deposition of such materials, the use of electrolytes that include cyanide compounds has been proposed. For example, Soviet Union Patent Application 377 435 A discloses a copper-tin alloy that is electrolytically deposited from a bath containing copper (I) cyanide, potassium cyanide, sodium stannate, sodium hydroxide and 3-methylbutanol. This electrolyte composition, however, has a very high cyanide concentration, making general handling as well as waste treatment hazardous.
Alternatives to co-deposition of such tin alloys by electroplating are known. For example, U.S. Pat. No. 6,476,494 to Hur et al discloses formation of silver-tin alloy solder bumps by electroplating silver on exposed portions of underbump metallurgy, plating tin on the silver, and reflowing the structure to form silver-tin alloy solder bumps. Composition of the silver-tin alloy is difficult to precisely control in this process, as it depends on a number of variables which themselves must be accurately controlled. For example, the amount of silver that diffuses into the tin and thus silver concentration is a function of reflow temperature, reflow time, silver and tin layer thicknesses, as well as other parameters. Another proposed alternative to co-deposition of tin alloys involves tin electroplating followed by exchange plating of the alloying metal and a reflow process. Such a method typically requires a significant process time, and precise control of the alloy concentration can be difficult.
There is thus a continuing need in the art for electroplating compositions for depositing tin alloys on a substrate, which are substantially free of lead and cyanides, and form alloys having good mechanical properties, are easily solderable, and can be electrolytically co-deposited. There further is a need for such electroplating compositions that can be used for the formation of interconnect bumps on a semiconductor device for wafer-level-packaging purposes.